1. Field of the Invention
The present invention relates to a heart-function monitor apparatus which monitors a function of the heart of a living subject by evaluating a cardiac mechanical efficiency, i.e., a mechanical efficiency of the heart.
2. Related Art Statement
When a characteristic of the left ventricle of the heart as an elastic tube, that is, an elastic coefficient of the same, at a telesystolic time immediately before the aortic valve is closed, is defined as a left-ventricle telesystolic elastance E.sub.es, and a characteristic of the aorta as an effective elastic tube, that is, an elastic coefficient of the aorta is defined as an aorta effective elastance E.sub.a, the ratio, E.sub.es /E.sub.a, of the left-ventricle telesystolic elastance E.sub.es to the aorta effective elastance E.sub.a indicates a degree of balance of the connection of the left ventricle and the aorta, that is, a mechanical efficiency of the left ventricle. Accordingly, this ratio E.sub.es /E.sub.a can be used as an important index of the function of the heart. It has been theoretically and experimentally elucidated that the ratio E.sub.es /E.sub.a changes depending upon the current state of the cardiac function, such as at rest, under stress, or in heart failure, and that the ratio E.sub.es /E.sub.a reflects cardiac metabolic rate (i.e., the ratio of the amount of work of the heart to the amount of consumption of oxygen by the cardiac muscle).
However, determination of the above left-ventricle telesystolic elastance E.sub.es, which is also known as the maximum pressure-volume ratio, or the left-ventricle telesystolic pressure-volume ratio, needs (a) detecting continuously respective changes of the inner pressure and inner volume of the left ventricle, (b) obtaining, in a two-dimensional coordinate system having a volume axis indicative of the inner volume of the left ventricle and a pressure axis indicative of the inner pressure of the same, a plurality of pressure-volume loops before and after preload or afterload is applied to the cardiac muscle, (c) estimating, based on the plurality of pressure-volume loops, a left-ventricle unstressed volume, V.sub.0, taken when the inner pressure would take zero, and (d) determining the telesystolic elastance E.sub.es by dividing a telesystolic pressure, P.sub.es, by the difference of a telesystolic volume, V.sub.es, and the unstressed volume V.sub.0. Thus, the determination of the telesystolic elastance E.sub.es needs measuring simultaneously the inner pressure and inner volume of the left ventricle. Conventionally, this determination has been carried out by an invasive method in which a cutting operation or a catheter insertion is employed. Thus, it has been very difficult to monitor the cardiac function. In addition, determination of the aorta effective elastance E.sub.a needs (e) determining, in the above-indicated two-dimensional coordinate system, the effective elastance E.sub.a by dividing the telesystolic pressure P.sub.es by the difference of a telediastolic volume, V.sub.ed, and the telesystolic volume V.sub.es. Thus, conventionally, this determination also needs measuring invasively the inner pressure and inner volume of the left ventricle, and it has been very difficult to monitor the cardiac function.